Deep vein thrombosis, acute pulmonary embolism, myocardial infarction, unstable angina, arterial embolism, extra cerebral bleeding are various diseases requiring intravenous or subcutaneous heparin, either under acute or chronic way. Heparin administration is also indicated for the prevention of venous thrombosis that may arise during prolonged bed rest. This is true for patients with heart failure, or who have suffered from a heart attack or stroke. Patients subjected to major surgery, such as orthopaedic procedures as insertion of a prosthetic hip or knee, surgery involving cardiopulmonary bypass circuits, also benefit from treatment with UnFractionated Heparin (UFH).
In this clinical context UFH has been widely used for more than six decades thanks to its anticoagulant and antithrombotic activities. The mechanism of the anticoagulant action of this heparin has been extensively studied and largely explained. It acts by binding to antithrombin III present in blood plasma and by forming a complex that inhibits thrombin and coagulation factors IXa, Xa, and XIa. Structure and chemical properties of UFH have been elucidated in detail. It is a sulfated glycosaminoglycan composed of macromolecules with a molecular weight varying from 3,000 to 30,000 Da. When administered intravenously, UFH shows complex pharmacokinetics with anticoagulant action which cannot be precisely predicted and which differs considerably among patients. The dose-response relationship for UFH is nonlinear. Also, patient's response depends upon several factors such as: age, gender, body weight, smoking status, and renal function. As a consequence in clinical practice, it is very difficult to achieve a precise control of the therapeutic anticoagulation. UFH overdoses are therefore not rare in clinic. Administration of UFH requires a close monitoring of hemostasis via analysis of activated partial thromboplastin time (aPTT) or a specific dosage of factor Xa/IIa. Although well tolerated in most cases, the administration of UFH can lead to various side effects, such as haemorrhage, thrombocytopenia and osteoporosis. In the mid-1980s low-molecular-weight heparins (LMWH) were introduced in clinic practice as antithrombotic drugs first to prevent postoperative deep vein thrombosis. LMWHs have molecular weights ranging from 2,000 to 10,000 Da. Recent studies have highlighted that LMWHs cause fewer side effects, although, at the same time, demonstrating low pharmacological potency compared to UFH.
In contrast to some chronic side effects of heparin, haemorrhage requires the rapid neutralization of heparin by the administration of an antidote. Until now protamine sulfate (PS) is the only drug available on the market to counteract quickly and efficiently the action of heparin. The European Pharmacopoeia monograph defines protamine sulphate as consisting of the sulphate of basic peptides extracted from the sperm or roe of fish, usually species of Salmonidae and Clupeidae. Upon injection within blood PS forms stable polyelectrolyte complexes with heparin and reverses its anticoagulant activity.
A disadvantage of PS arises from its natural origin which causes several limitations on the final characteristics of the product, such as a lack of control of its molecular features (primary amino sequence, molecular weight). For example when subjected to reversed phase high performance liquid chromatography (RP-HPLC) salmon protamine appears as a mixture comprising four main components accompanied by a number of minor species. This purity level, but also its natural source represents therefore a potent risk of persistence of residues, such as heavy metals, endotoxins and other antigenic biological contaminants.
The availability issue is a further disadvantage of PS, as indicated in a recent report of the European Medical Agency. Indeed with the recent fishing restrictions in Japan following the earthquake and the tsunami in March 2011, sourcing of the raw material was done in other fishing grounds and the new natural raw material has shown endogenous heterogeneity. The European Medical Agency was informed by member states, as well as from market authorisation holders that a potential supply shortage of the protamine sulphate containing medicinal products may occur shortly.
There have been research efforts to identify potential alternatives to protamine sulfate. For example salicylamide derivatives, water-soluble chitosan, or low molecular weight protamine have been tested. However, no compound other than PS is currently admitted in clinic to neutralize heparin. The literature has not reported a drug more potent and safer than protamine. Heparinase for example after clinical trials has proved to be more risky than PS.
In US2012308546, J. N. Kizhakkedathu et al. reported the neutralization of heparin (UFH and LMWH) with synthetic polymers consisting in a hyperbranched polyglycerol core grafted with polyvalent primary amino-groups. However, these polymers are complex macromolecular structures which require several tedious steps for their synthesis. Moreover, high molecular weight polymers may have poor clearance capability.
In JP 2012 029831 and in “Bioconjugate Chemistry, vol 22, p 193”, Yasuhide et al. reported a heparin coating comprising a six-branched, star-shaped poly(2-(dimethylaminoethyl)-methacrylate) which provides antithrombogenicity. In that case, the synthetic polymer material is only used to fix heparin to the surface of a medical device in order to form a coating. With this coating, the antithrombogenicity of heparin is maintained and conferred to the surface.
Other synthetic polymers bearing amino groups such as polyethylene imine (PEI) and Polybrene have also been tested as heparin antidotes but the properties of these synthetic polycations are not favourable for pharmaceutical application. In particular, PEI has an extensive branched structure resulting in a compact structure which does not promote interaction with heparin.